1. Field of invention
This invention relates to the method and the device of calculating aircraft braking friction and other aircraft performance and pavement surface characteristics parameters related to aircraft landing and takeoff including but not limited to aircraft braking action, aircraft takeoff distance, aircraft landing distance, runway surface conditions and runway surface friction—from now on referred as true aircraft landing performance parameters—based on the data collected or otherwise available on board of an aircraft in electronic or other format from the aircraft Flight Data Recorder (FDR) or any other flight data providing or management system for example the Quick Access Recorder (QAR).
2. Background
Under severe winter conditions airlines, airports, civil aviation organizations and countries rigorously impose limits on aircraft takeoff, landing and other surface movement operations as well as enforce weight penalties for aircraft takeoffs and landings. These limits depend on the weather, runway and taxiway surface conditions and aircraft braking and takeoff performance. At the present these limits are calculated from the assumed aircraft braking performance based on runway conditions. These conditions are established by visual inspections, weather reports and the measurements of runway friction coefficient using ground friction measurement equipment.
At the present time, there are several practices to calculate the assumed aircraft braking performance:
1. The Canadian CRFI method:
The CRFI method comprises of a runway surface friction measurement performed by braking a passenger vehicle traveling on the runway at a certain speed and measuring the maximum deceleration of it at several locations along the length of the runway. The measured deceleration data is taken then and a braking index chart is used to calculate the assumed aircraft braking performance. The obtained aircraft landing performance data and calculated assumed braking friction is provided to airline operators, pilots and airport personnel for decision making.
2. The reported runway friction coefficient by a runway friction measurement equipment.
There are a great many number of runway friction measurement devices manufactured by different companies, in different countries and working based on different principles. Some of the most common devices are: (a) continuous friction measurement equipment (CFME); (b) decelerometers; (c) side force friction coefficient measurement equipment. These equipment are operated by airport operation personnel according to the manufacturer's instructions on the runways, aprons, and taxiways and the measured friction coefficient is recorded. The recorded friction coefficient is then distributed to airline operation personnel, pilots, and airport personnel. The measured coefficient of friction is dependent of the measurement device, under the same conditions and on the same runway different runway friction measurement devices based on different principles will record different runway friction coefficients. These runway friction coefficients are assumed to relate to actual aircraft landing and takeoff performance.
3. The new proposed IRFI method:
The International Runway Friction Index (IRFI) is a computational method to harmonize the reported runway friction numbers reported by the many different runway surface friction measurement equipments. The method was developed through an international effort with 14 participating countries. The method is a mathematical procedure based on simple linear correlations. The IRFI procedure is using a mathematical transformation to take the reported measurement of a runway friction measurement device and compute using simple mathematical methods an index called the IRFI. The mathematical procedures are the same for all the different runway friction measurement device using a different set of constant parameters that was determined for each individual device. It is assumed that using this procedure the different runway friction measurement devices reporting different friction coefficients can be harmonized. The calculated IRFI is assumed to correlate to aircraft landing and takeoff performance.
4. Pre-determined friction levels based on observed runway conditions, current and forecasted weather conditions:
This method is available for airport operators according to new regulations. The method is based on airport personnel driving through the runway and personally observing the runway surface conditions. The ice, snow, water and other possible surface contaminants are visually observed and their depth measured or estimated by visual observation. The estimated runway conditions with weather information are then used to lookup runway friction coefficient in a table.
All these above mentioned practices are based on the measurement of the runway friction coefficient using ground friction measuring equipment, visual observation, weather information or combinations of these. However, according to present practices, there are several problems with the measurement of the runway friction coefficient using these methods.
1. Need of a special device/car: there is a special car needed to be able to measure the runway friction coefficient. There are special devices to measure the runway friction coefficient that are commercially available; however, most of these devices are very expensive. Therefore, not every airport can afford to have one.
2. Close of runway: for the duration of the measurement the runway has to be closed for takeoffs and landings as well as any aircraft movement. The measurement of the runway surface friction takes a relatively large amount of time since a measuring device has to travel the whole length of the runway at a minimum one time but during severe weather conditions it is possible that more than one measurement run is needed to determine runway surface friction. The closing of an active runway causes the suspension of takeoff and landing aircraft operations for a lengthened period of time and therefore is very costly for both the airlines and the airport. The using of ground vehicles to measure runway friction poses safety hazard especially under severe weather conditions.
3. Inaccurate result due to lack of maintenance and inaccurate calibration level: the result of the measurements are very dependent of the maintenance and the calibration level of measurement devices, therefore the result can vary much, and could loose reliability.
4. Confusing results due to the differences between ground friction devices: It has been established that the frictional values reported by different types of ground friction measurement equipment are substantially different. In fact, the same type and manufacture, and even the same model of equipment frequently report highly scattered frictional data. Calibration and measurement procedures are different for different types of devices. The repeatability and reproducibility scatter or in other word uncertainty of measurements for each type of ground friction measurement device is therefore amplified and the spread of friction measurement values among different equipment types is significant.
5. Inaccurate result due to rapid weather change: Airport operation personnel, in taking on the responsibility of conducting friction measurements during winter storms, find it difficult to keep up with the rapid changes in the weather. During winter storms runway surface conditions can change very quickly and therefore friction measurement results can become obsolete in a short amount of time, thus misrepresenting landing and takeoff conditions.
6. Inaccurate result due to the difference between aircraft and the ground equipment: It is proven that the aircraft braking friction coefficients of contaminated runways are different for aircrafts compared to those reported by the ground friction measurement equipment.
7. Inaccurate result due to the lack of uniform runway reporting practices: For many years the international aviation community has had no uniform runway friction reporting practices. The equipment used and procedures followed in taking friction measurements varies from country to country. Therefore, friction readings at various airports because of differences in reporting practices may not be reliable enough to calculate aircraft braking performance.
Therefore this invention recognizes the need for a system directly capable of determining the true aircraft landing performance parameters based on the data collected by and available in the aircraft Flight Data Recorder (FDR) or other flight data management systems. By utilizing the novel method in this invention for the first time every involved personnel in the ground operations of an airport and airline operations including but not limited to aircraft pilots, airline operation officers and airline managers as well as airport operators, managers and maintenance crews, will have the most accurate and most recent information on runway surface friction and aircraft braking action, especially on winter contaminated and slippery runways.
Utilizing this method the aviation industry no longer has to rely on different friction reading from different instrumentations and from different procedures.
Therefore, this method will represent a direct and substantial benefit for the aviation industry.